The race among smartphone operating system vendors looks either like a close one, or one with a clear winner, according to a leading analytics company.

In November, for example, among U.S. users who had bought a smartphone in the previous six months, Android was well ahead of competitors.

However, the picture appears quite different when all U.S. smartphone users are considered.

In July, Google's (NASDAQ: GOOG) Android seemed stuck in the pack with 28 percent share of users who'd bought a smartphone in the preceding six months, according to the latest report from tracking firm
The Nielsen Company.

At the time, Apple's (NASDAQ: APPL) IOS came in at 26.2 percent, and RIM's (NASDAQ: RIMM) BlackBerry with 25.3 percent.